Network Working Group M. Thomson
Internet-Draft Mozilla
Intended status: Informational January 02, 2019
Expires: July 6, 2019
Long-term Viability of Protocol Extension Mechanisms
draft-thomson-use-it-or-lose-it-03
Abstract
The ability to change protocols depends on exercising the extension
and version negotiation mechanisms that support change. Protocols
that don't use these mechanisms can find that deploying changes can
be difficult and costly.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Implementations of Protocols are Imperfect . . . . . . . . . 3
2.1. Good Protocol Design is Not Sufficient . . . . . . . . . 3
2.2. Multi-Party Interactions and Middleboxes . . . . . . . . 5
3. Retaining Viable Protocol Evolution Mechanisms . . . . . . . 6
3.1. Examples of Active Use . . . . . . . . . . . . . . . . . 6
3.2. Dependency is Better . . . . . . . . . . . . . . . . . . 7
3.3. Unused Extension Points Become Unusable . . . . . . . . . 7
4. Defensive Design Principles for Protocols . . . . . . . . . . 8
4.1. Active Use . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Grease . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Cryptography . . . . . . . . . . . . . . . . . . . . . . 9
4.4. Effective Feedback . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Informative References . . . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
A successful protocol [SUCCESS] will change in ways that allow it to
continue to fulfill the needs of its users. New use cases,
conditions and constraints on the deployment of a protocol can render
a protocol that does not change obsolete.
Usage patterns and requirements for a protocol shift over time.
Protocols can react to these shifts in one of three ways: adjust
usage patterns within the constraints of the protocol, extend the
protocol, and replace the protocol. These reactions are
progressively more disruptive, but are also dictated by the nature of
the change in requirements over longer periods.
Experience with Internet-scale protocol deployment shows that
changing protocols is not uniformly successful. [TRANSITIONS]
examines the problem more broadly.
This document examines the specific conditions that determine whether
protocol maintainers have the ability to design and deploy new or
modified protocols. Section 4 outlines several strategies that might
aid in ensuring that protocol changes remain possible over time.
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2. Implementations of Protocols are Imperfect
A change to a protocol can be made extremely difficult to deploy if
there are bugs in implementations with which the new deployment needs
to interoperate. Bugs in the handling of new codepoints or
extensions can mean that instead of handling the mechanism as
designed, endpoints react poorly. This can manifest as abrupt
termination of sessions, errors, crashes, or disappearances of
endpoints and timeouts.
Interoperability with other implementations is usually highly valued,
so deploying mechanisms that trigger adverse reactions like these can
be untenable. Where interoperability is a competitive advantage,
this is true even if the negative reactions happen infrequently or
only under relatively rare conditions.
Deploying a change to a protocol could require fixing a substantial
proportion of the bugs that the change exposes. This can involve a
difficult process that includes identifying the cause of these
errors, finding the responsible implementation, coordinating a bug
fix and release plan, contacting the operator of affected services,
and waiting for the fix to be deployed to those services.
Given the effort involved in fixing these problems, the existence of
these sorts of bugs can outright prevent the deployment of some types
of protocol changes. It could even be necessary to come up with a
new protocol design that uses a different method to achieve the same
result.
The set of interoperable features in a protocol is often the subset
of its features that have some value to those implementing and
deploying the protocol. It is not always the case that future
extensibility is in that set.
2.1. Good Protocol Design is Not Sufficient
It is often argued that the design of a protocol extension point or
version negotiation capability is critical to the freedom that it
ultimately offers.
RFC 6709 [EXTENSIBILITY] contains a great deal of well-considered
advice on designing for extension. It includes the following advice:
This means that, to be useful, a protocol version- negotiation
mechanism should be simple enough that it can reasonably be
assumed that all the implementers of the first protocol version at
least managed to implement the version-negotiation mechanism
correctly.
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This has proven to be insufficient in practice. Many protocols have
evidence of imperfect implementation of these critical mechanisms.
Mechanisms that aren't used are the ones that fail most often. The
same paragraph from RFC 6709 acknowledges the existence of this
problem, but does not offer any remedy:
The nature of protocol version-negotiation mechanisms is that, by
definition, they don't get widespread real-world testing until
_after_ the base protocol has been deployed for a while, and its
deficiencies have become evident.
Indeed, basic interoperability is considered critical early in the
deployment of a protocol, and any engineering practice that values
simplicity will tend to make version negotiation and extension
mechanisms optional for this basic interoperability. This leads to
these mechanisms being uniquely affected by this problem.
Transport Layer Security (TLS) [TLS12] provides examples of where a
design that is objectively sound fails when incorrectly implemented.
TLS provides examples of failures in protocol version negotiation and
extensibility.
Version negotiation in TLS 1.2 and earlier uses the "Highest mutually
supported version (HMSV)" scheme exactly as it is described in
[EXTENSIBILITY]. However, clients are unable to advertise a new
version without causing a non-trivial proportions of sessions to fail
due to bugs in server and middlebox implementations.
Intolerance to new TLS versions is so severe [INTOLERANCE] that TLS
1.3 [TLS13] has abandoned HMSV version negotiation for a new
mechanism.
The server name indication (SNI) [TLS-EXT] in TLS is another
excellent example of the failure of a well-designed extensibility
point. SNI uses the same technique for extension that is used with
considerable success in other parts of the TLS protocol. The
original design of SNI includes the ability to include multiple names
of different types.
What is telling in this case is that SNI was defined with just one
type of name: a domain name. No other type has ever been
standardized, though several have been proposed. Despite an
otherwise exemplary design, SNI is so inconsistently implemented that
any hope for using the extension point it defines has been abandoned
[SNI].
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2.2. Multi-Party Interactions and Middleboxes
Even the most superficially simple protocols can often involve more
actors than is immediately apparent. A two-party protocol has two
ends, but even at the endpoints of an interaction, protocol elements
can be passed on to other entities in ways that can affect protocol
operation.
One of the key challenges in deploying new features in a protocol is
ensuring compatibility with all actors that could influence the
outcome.
Protocols deployed without active measures against intermediation
will tend to become intermediated over time, as network operators
deploy middleboxes to perform some function on traffic. In
particular, one of the consequences of an unencrypted protocol is
that any element on path can interact with the protocol. For
example, HTTP was specifically designed with intermediation in mind,
transparent proxies [HTTP] are not only possible but sometimes
advantageous, despite some significant downsides. Consequently,
transparent proxies for cleartext HTTP are commonplace.
Middleboxes are also protocol participants, to the degree that they
are able to observe and act in ways that affect the protocol. The
degree to which a middlebox participates varies from the basic
functions that a router performs to full participation. For example,
a SIP back-to-back user agent (B2BUA) [B2BUA] can be very deeply
involved in the SIP protocol.
This phenomenon appears at all layers of the protocol stack, even
when protocols are not designed with middlebox participation in mind.
TCP's [TCP] extension points have been rendered difficult to use,
largely due to middlebox interactions, as experience with Multipath
TCP [MPTCP] has shown. IP's version field was rendered useless when
encapsulated over Ethernet, requring a new ethertype with IPv6
[RFC2462], due in part to layer 2 devices making version-independent
assumptions about the structure of the IPv4 header.
By increasing the number of different actors involved in any single
protocol exchange, the number of potential implementation bugs that a
deployment needs to contend with also increases. In particular,
incompatible changes to a protocol that might be negotiated between
endpoints in ignorance of the presence of a middlebox can result in a
middlebox acting badly.
Thus, middleboxes can increase the difficulty of deploying changes to
a protocol considerably.
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3. Retaining Viable Protocol Evolution Mechanisms
The design of a protocol for extensibility and eventual replacement
[EXTENSIBILITY] does not guarantee the ability to exercise those
options. The set of features that enable future evolution need to be
interoperable in the first implementations and deployments of the
protocol. Active use of mechanisms that support evolution is the
only way to ensure that they remain available for new uses.
3.1. Examples of Active Use
The conditions for retaining the ability to evolve a design is most
clearly evident in the protocols that are known to have viable
version negotiation or extension points. The definition of
mechanisms alone is insufficient; it's the active use of those
mechanisms that determines the existence of freedom.
For example, header fields in email [SMTP], HTTP [HTTP] and SIP [SIP]
all derive from the same basic design. There is no evidence of
significant barriers to deploying header fields with new names and
semantics in email and HTTP, though the widespread deployment of SIP
B2BUAs means that new SIP header fields can be more difficult.
In another example, the attribute-value pairs (AVPs) in Diameter
[DIAMETER] are fundamental to the design of the protocol. The
definition of new uses of Diameter regularly exercise the ability to
add new AVPs and do so with no fear that the new feature might not be
successfully deployed.
These examples show extension points that are heavily used also being
relatively unaffected by deployment issues preventing addition of new
values for new use cases.
These examples also confirm the case that good design is not a
prerequisite for success. On the contrary, success is often despite
shortcomings in the design. For instance, the shortcomings of HTTP
header fields are significant enough that there are ongoing efforts
to improve the syntax [HTTP-HEADERS].
Only using a protocol capability is able to ensure availability of
that capability. Protocols that fail to use a mechanism, or a
protocol that only rarely uses a mechanism, suffer an inability to
rely on that mechanism.
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3.2. Dependency is Better
The best way to guarantee that a protocol mechanism is used is to
make it critical to an endpoint participating in that protocol. This
means that implementations rely on both the existence of the protocol
mechanism and its use.
For example, the message format in SMTP relies on header fields for
most of its functions, including the most basic functions. A
deployment of SMTP cannot avoid including an implementation of header
field handling. In addition to this, the regularity with which new
header fields are defined and used ensures that deployments
frequently encounter header fields that it does not understand. An
SMTP implementation therefore needs to be able to both process header
fields that it understands and ignore those that it does not.
In this way, implementing the extensibility mechanism is not merely
mandated by the specification, it is crucial to the functioning of a
protocol deployment. Should an implementation fail to correctly
implement the mechanism, that failure would quickly become apparent.
Caution is advised to avoid assuming that building a dependency on an
extension mechanism is sufficient to ensure availability of that
mechanism in the long term. If the set of possible uses is narrowly
constrained and deployments do not change over time, implementations
might not see new variations or assume a narrower interpretation of
what is possible. Those implementations might still exhibit errors
when presented with a new variation.
3.3. Unused Extension Points Become Unusable
In contrast, there are many examples of extension points in protocols
that have been either completely unused, or their use was so
infrequent that they could no longer be relied upon to function
correctly.
HTTP has a number of very effective extension points in addition to
the aforementioned header fields. It also has some examples of
extension point that are so rarely used that it is possible that they
are not at all usable. Extension points in HTTP that might be unwise
to use include the extension point on each chunk in the chunked
transfer coding [HTTP], the ability to use transfer codings other
than the chunked coding, and the range unit in a range request
[HTTP-RANGE].
Even where extension points have multiple valid values, if the set of
permitted values does not change over time, there is still a risk
that new values are not tolerated by existing implementations. If
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the set of values for a particular field remains fixed over a long
period, some implementations might not correctly handle a new value
when it is introduced. For example, implementations of TLS broke
when new values of the signature_algorithms extension were
introduced.
4. Defensive Design Principles for Protocols
There are several potential approaches that can provide some measure
of protection against a protocol deployment becoming resistant to
change.
4.1. Active Use
As discussed in Section 3, the most effective defense against misuse
of protocol extension points is active use.
Implementations are most likely to be tolerant of new values if they
depend on being able to use new values. Failing that,
implementations that routinely see new values are more likely to
correctly handle new values. More frequent changes will improve the
likelihood that incorrect handling or intolerance is discovered and
rectified. The longer an intolerant implementation is deployed, the
more difficult it is to correct.
What active use means could depend greatly on the environment in
which a protocol is deployed. The frequency of changes necessary to
safeguard some mechanisms might be slow enough to attract
ossification in another protocol deployment, while being excessive in
others. There are currently no firm guidelines for new protocol
development, as much is being learned about what techniques are most
effective.
4.2. Grease
"Grease" [GREASE] identifies lack of use as an issue (protocol
mechanisms "rusting" shut) and proposes a system of use that
exercises extension points by using dummy values.
The primary feature of the grease design is aimed at the style of
negotiation most used in TLS, where the client offers a set of
options and the server chooses the one that it most prefers from
those that it supports. A client that uses grease randomly offers
options - usually just one - from a set of reserved values. These
values are guaranteed to never be assigned real meaning, so the
server will never have cause to genuinely select one of these values.
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The principle that grease operates on is that an implementation that
is regularly exposed to unknown values is not likely to become
intolerant of new values when they appear. This depends largely on
the assumption that the difficulty of implementing the protocol
mechanism correctly is not significantly more effort than
implementing code to specifically filter out the randomized grease
values.
To avoid simple techniques for filtering greasing codepoints, grease
values are not reserved from a single contiguous block of code
points, but are distributed evenly across the entire space of code
points. Reserving a randomly selected set of code points has a
greater chance of avoiding this problem, though it might be more
difficult to specify and implement, especially over larger code point
spaces.
Without reserved greasing codepoints, an implementation can use code
points from spaces used for private or experimental use if such a
range exists. In addition to the risk of triggering participation in
an unwanted experiment, this can be less effective. Incorrect
implementations might still be able to correctly identify these code
points and ignore them.
Grease is deployed with the intent of quickly detecting errors in
implementing the mechanisms it safeguards. Though it has been
effective at revealing problems in some cases with TLS, its efficacy
isn't proven more generally.
This style of defensive design has some limitations. It does not
necessarily create the need for an implementation to rely on the
mechanism it safeguards; that is determined by the underlying
protocol itself. More critically, it does not easily translate to
other forms of extension point. For instance, HMSV negotiation
cannot be greased in this fashion. Other techniques might be
necessary for protocols that don't rely on the particular style of
exchange that is predominant in TLS.
4.3. Cryptography
Cryptography can be used to reduce the number of entities that can
participate in a protocol. Using tools like TLS ensures that only
authorized participants are able to influence whether a new protocol
feature is used.
Permitting fewer protocol participants reduces the number of
implementations that can prevent a new mechanism from being deployed.
As recommended in [PATH-SIGNALS], use of encryption and integrity
protection can be used to limit participation.
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For example, the QUIC protocol [QUIC] adopts both encryption and
integrity protection. Encryption is used to carefully control what
information is exposed to middleboxes. For those fields that are not
encrypted, QUIC uses integrity protection to prevent modification.
4.4. Effective Feedback
While not a direct means of protecting extensibility mechanisms,
feedback systems can be important to discovering problems.
Visibility of errors is critical to the success of the grease
technique (see Section 4.2). The grease design is most effective if
a deployment has a means of detecting and reporting errors. Ignoring
errors could allow problems to become entrenched.
Feedback on errors is more important during the development and early
deployment of a change. It might also be helpful to disable
automatic error recovery methods during development.
Automated feedback systems are important for automated systems, or
where error recovery is also automated. For instance, connection
failures with HTTP alternative services [ALT-SVC] are not permitted
to affect the outcome of transactions. An automated feedback system
for capturing failures in alternative services is therefore necessary
for failures to be detected.
5. Security Considerations
The ability to design, implement, and deploy new protocol mechanisms
can be critical to security. In particular, it is important to be
able to replace cryptographic algorithms over time [AGILITY]. For
example, preparing for replacement of weak hash algorithms was made
more difficult through misuse [HASH].
6. IANA Considerations
This document makes no request of IANA.
7. Informative References
[AGILITY] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
.
[ALT-SVC] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, .
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[B2BUA] Kaplan, H. and V. Pascual, "A Taxonomy of Session
Initiation Protocol (SIP) Back-to-Back User Agents",
RFC 7092, DOI 10.17487/RFC7092, December 2013,
.
[DIAMETER]
Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
Ed., "Diameter Base Protocol", RFC 6733,
DOI 10.17487/RFC6733, October 2012,
.
[EXTENSIBILITY]
Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
DOI 10.17487/RFC6709, September 2012,
.
[GREASE] Benjamin, D., "Applying GREASE to TLS Extensibility",
draft-ietf-tls-grease-01 (work in progress), June 2018.
[HASH] Bellovin, S. and E. Rescorla, "Deploying a New Hash
Algorithm", Proceedings of NDSS '06 , 2006,
.
[HTTP] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
.
[HTTP-HEADERS]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-09 (work in progress),
December 2018.
[HTTP-RANGE]
Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
RFC 7233, DOI 10.17487/RFC7233, June 2014,
.
[INTOLERANCE]
Kario, H., "Re: [TLS] Thoughts on Version Intolerance",
July 2016, .
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[MPTCP] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
.
[PATH-SIGNALS]
Hardie, T., "Transport Protocol Path Signals", draft-iab-
path-signals-02 (work in progress), November 2018.
[QUIC] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-17 (work
in progress), December 2018.
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, DOI 10.17487/RFC2462,
December 1998, .
[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
.
[SMTP] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
.
[SNI] Langley, A., "Accepting that other SNI name types will
never work", March 2016,
.
[SUCCESS] Thaler, D. and B. Aboba, "What Makes for a Successful
Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
.
[TCP] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
.
[TLS-EXT] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
.
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[TLS12] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018.
[TRANSITIONS]
Thaler, D., Ed., "Planning for Protocol Adoption and
Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
May 2017, .
Acknowledgments
Mirja Kuehlewind, Mark Nottingham, and Brian Trammell made
significant contributions to this document.
Author's Address
Martin Thomson
Mozilla
Email: mt@lowentropy.net
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